Two-way radios, cell phones, and other handheld communication devices are often used in flammable or potentially explosive areas. Radios intended for hazardous or potentially explosive environments may require “intrinsically safe” type certification through an agency such as Factory Mutual (FM) or be ATEX compliant (from the French—ATmosphere Explosibles) in the European Union. When dealing with battery powered radios, the prevention of sparking at the contacts located between the battery and the radio is necessary to meet intrinsically safe operating requirements.
Certain circuit designs, such as radio pulse transmitters, are of particular concern when dealing with intrinsically safe operation due to the large capacitors required to smooth peak current to the transmitter power amplifier supply as well as filter capacitance for other circuits. Typically, capacitance values of 1000 uF to 5000 uF may be required to accomplish such smoothing and may be distributed throughout the circuit. Large capacitance is problematic in intrinsically safe systems due to: (1) the large amount of energy required to charge the radio capacitance upon attachment (inrush); and (2) the large amount of energy remaining in the capacitance in the radio and exposed at the contacts if the battery is removed from the radio and short circuited. Upon attachment to the radio, the energy in the battery charges this capacitance quickly, creating the potential for a spark to be generated. Likewise, when the battery is removed from the radio, the energy stored in the radio could be rapidly discharged if the radio contacts were to become shorted before the energy could otherwise be dissipated. A spark in a hazardous environment, generated from either of these scenarios, could cause an explosion. None of the currently available approaches to intrinsic safety address the possible short circuit discharge conditions on the exposed radio contacts.
Accordingly, there is a need for an improved intrinsically safe circuit and manner of providing intrinsically safe circuit operation of a battery powered communication device.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.